Agricultural implements having one or more ground engaging tools, such as opener disks and seed drills, are well known. A conventional agricultural implement generally includes a frame to be conveyed as a trailer behind a tractor or similar self propelled vehicle. A rockshaft, a tool bar or a similarly moveable member is mounted to the frame so as to support one or more such ground engaging tools. Movement of the member, and thus the tools, typically is accomplished using at least one hydraulic cylinder operatively extending between the frame and the moveable member. As such, the moveable member can be lowered so as to engage the tools with the ground.
It is desirable to control the degree or depth in which the ground engaging tool engages the ground. Improper tool depth will adversely impact tillage and seeding results, including uneven seed germination, inconsistent crop maturity and potentially lower crop yields. However, tool depth will vary not only due to irregularities and changes in the surface and type of soil across which the agricultural implement is conveyed, but also due to variations in the overall load imparted by the agricultural implement and materials loaded thereon during operation. As such, it is preferable to enable constant adjustment of the tool depth for optimal results.
A variety of techniques have been developed to address tool depth control. For example, it is common to mount each ground engaging tool for independent movement relative to the other ground engaging tools. A biasing device, such as a mechanical or hydraulic spring is provided to apply a continuous downwardly-directed force on each ground engaging tool. Although such independently-biased mounting configurations satisfactorily accommodate minor fluctuations for each ground engaging tool, it often is necessary to apply and adjust an overall downwardly-directed force acting against the entire structure upon which the ground engaging tools are mounted. In this manner, greater control over a more diverse range of operating conditions can be accommodated.
One known technique of applying an overall downwardly-directed force is to load separate weights on the agricultural implement. This technique, however, is labor intensive and often provides limited results.
It also is known to use one or more hydraulic cylinders or similar devices for tool depth control. This may be accomplished either by implementing the same hydraulic cylinders that are used to raise and lower the tools between their storage and operating positions, respectively, or by using separate hydraulic cylinders for such depth control. A variety of hydraulic circuits have been developed to operate these hydraulic cylinders so as to control the downward force exerted to the tools, and thus control the depth in which the tools engage the ground. These hydraulic circuits generally are connected in fluid communication via a tractor auxiliary control valve to a pump or similar hydraulic source provided on the tractor.
One known hydraulic circuit used for tool depth control includes a check valve located along a hydraulic supply line extending between the hydraulic pump and the head side of the hydraulic cylinder. Generally, pressurized hydraulic flow from the pump is directed downstream across the check valve to the head side of the hydraulic cylinder. The hydraulic cylinder is thereby extended so as to urge the tool into engagement with the ground as a function of the pressure within the head side of the hydraulic cylinder. Once the desired pressure of the hydraulic cylinder is reached, pressurized flow from the pump is secured, typically by using the tractor auxiliary control valve, and static pressure is maintained downstream of the check valve.
To ensure that the hydraulic cylinder is not over pressurized, such as when the tool transverses an undulation in the ground and is thereby forced upward, a fixed pressure relief valve is interconnected between the check valve and the head side of the hydraulic cylinder. The pressure relief valve includes a return port which returns pressurized fluid to the supply side of the check valve. In this manner, and with the pump secured, and with the tractor auxiliary control valve in its float position, hydraulic fluid between the check valve and the head side of the hydraulic cylinder is returned to the tractor through the supply line when the fixed pressure of the pressure relief valve is exceeded. The head side of the hydraulic cylinder can be repressurized by operating the (tractor valve) pump to again direct pump pressurized hydraulic fluid across the check valve.
Independently, another check valve is separately connected to the rod side of the hydraulic cylinder. Pressurized hydraulic fluid is supplied across this separate check valve to the rod side of the hydraulic cylinder to retract the hydraulic cylinder. The hydraulic cylinder is then held in its retracted position due to the pressurization that is maintained within the rod side of the hydraulic cylinder due to the separate check valve. Pilot lines generally are interconnected between the two separate check valves such that the check valve connected to the rod side of the hydraulic cylinder is opened when pressurized hydraulic flow is directed to the head side of the hydraulic cylinder, and the check valve connected to the head side of the hydraulic cylinder is opened when pressurized hydraulic flow is directed to the rod side of the hydraulic cylinder.
Although this known hydraulic circuit provides satisfactory results, it is not capable of continuous down pressure operation such that the downward pressure acting against the ground engaging tool can be maintained continuously. Furthermore, because static pressure is used to urge the tools into engagement with the ground, this hydraulic circuit lacks versatility and certain adjustment capabilities.
As an alternative, it is well known to use a pressure reducing valve in communication between a hydraulic pump and the head side of a hydraulic cylinder. The pressure reducing valve is provided to operate and maintain the hydraulic cylinder at a predetermined pressure. Although the pressure reducing valve is intended to allow for continuous down pressure operation, certain disadvantages are associated with this fluid circuit. For example, it has been found that the tractor auxiliary control valve will disengage from its selected operating position when excessive pressure in the hydraulic circuit is experienced or when the pressure reducing valve closes momentarily during system operation. It therefore is necessary to monitor and manually reset the operating position of the tractor auxiliary control valve during field use, or to manipulate or otherwise disable the tractor auxiliary control valve so as to prevent disengagement of the selected operating position.
In view of the above, there remains a need for a hydraulic circuit that allows continuous down pressure operation to continuously maintain a down pressure force on a ground engaging tool for enhanced tool depth control. Furthermore, it is desirable to provide a hydraulic circuit that allows either high pressure relief operation or continuous down pressure operation of the down pressure tool.